The present invention relates to power tools and in particular to a power tool having a microcomputer-based control circuit for monitoring and controlling various operating conditions and parameters.
Conventional power tools typically provide a few rudimentary mechanical controls for controlling the operation of the tool. In addition, very little information concerning the operating condition of the tool is generally communicated back to the operator.
Accordingly, it is the primary object of the present invention to provide a power tool that includes a microcomputer-based control circuit that is adapted to control various operating functions and continuously monitor certain critical operating parameters. In general, the control circuit is comprised of a microcomputer, a power semiconductor control device for controlling power to the motor, and one or more sensors for monitoring various parameters and providing feedback information in the microcomputer.
In the preferred embodiment, which describes the present invention in the environment of a drill press, the speed signal information derived from a speed sensor device operatively coupled to the motor is used by the microcomputer to control the speed of the motor, prevent motor kickback caused by a binding tool bit, detect a stall condition, and provide overload protection for the motor. In addition, the microcomputer in the preferred embodiment is programmed to gradually "ramp" the speed of the motor to the desired speed on start-up and when a faster speed setting is selected. The microcomputer is interfaced to input means, such as a keyboard, via which speed selections are entered by the operator. The microcomputer is programmed to control the speed of the motor by controlling the conduction angle of the power semiconductor control device in accordance with the desired speed value entered on the keyboard. In the event the speed of the motor is slowed through increased loading of the tool bit, the microcomputer is programmed to automatically respond by increasing the conduction angle of the semiconductor control device until the actual speed of the motor as measured by the speed sensor equals the desired speed value, or the maximum output of the motor is reached.
The microcomputer also utilizes the speed signal information from the speed sensor to provide anti-kickback protection for the drill press. More particularly, the microcomputer is programmed to recognize when the rotational speed of the motor is decelerating and to detect when the rate of deceleration exceeds a predetermined rate. In response to the detection of such an occurrence, the microcomputer will automatically cut power to the motor. In addition, if the motor is re-started and the tool bit does not break free within a prescribed time period, the microcomputer is further programmed to detect the stall condition and again remove power to the motor. More generally, whenever a lack of spindle movement is detected over a prescribed period of time, the microcomputer is programmed to recognize the condition as a stall and interrupt power to the motor.
A sophisticated form of overload protection is also provided which warns of an impending overload condition, using solely the speed signal information derived from the speed sensor and the known conduction angle of the semiconductor control device. Accordingly, an incremental cost savings is realized. The overload calculation utilizes a numerical integrator, which in the preferred embodiment comprises a storage register whose contents are periodically updated by adding to or subtracting from the current value a "heating value" which is determined according to the conduction angle (voltage) and measured speed of the motor. The "heating values" assigned to each of the various operating ranges of the motor are empirically determined and stored in matrix form in the microcomputer. Positive "heating values" correspond to a net heat increase and negative values correspond to a net heat loss. Whenever the accumulated total in the storage register exceeds a predetermined value, an overload condition is signalled. The signal is removed when the total drops below a second lower predetermined value.
The preferred embodiment of the microcomputer-based control circuit of the present invention also includes a position encoder device for providing position information to the microcomputer concerning the vertical position of the tool bit relative to a preselected reference position. The position encoder is operatively coupled to the depth adjustment mechanism of a drill press so that its output signal varies in accordance with vertical movement of the housing assembly. The microcomputer is programmed to calculate the vertical position of the tool bit relative to a reference position and display the result on a digital display. The reference position is selected by depressing the "SET DEPTH" button on the keyboard which zeroes the display.
The digital display can also be utilized by the operator to display the rotational speed of the tool bit by depressing the appropriate display function key on the keyboard. In addition, various warnings and error codes are automatically displayed on the display to warn the operator of a problem or inform the operator of the cause for a shutdown.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :